This is my version of a recipe Sunny Anderson cooked on The Chew. Kalamata olives, any nut and any sort of green leafy vegetable will work as substitutes. This makes more pesto than you need for that amount of pasta but we eat pesto on toasted bread, chips, crackers, and put it in a lot of summer dishes. I am very free with garlic and onions, even so, they don't take over this dish.
Bring a pot of salted water to a boil, blanch the spinach just till tender, about 1 minute. Drain and get rid of excess water. Put in processor bowl.
Heat a dry skillet. Add walnuts and toast, stirring and keeping an eye on them so they don't burn. Add to processor.
Add basil, garlic, Parmesan, and olives to processor. Pulse until well blended. Remove the center tube, turn the processor on and drizzle oil in until you get a good consistency. Taste, add salt if needed. Add pepper to taste.
Cook pasta according to manufacturers directions.
While pasta is cooking, heat oil in a skillet over medium heat. Add onions, lightly sautéing for about 1 minute. Add mushrooms and garlic, cooking till mushrooms are wilted.
Pour pasta into a bowl, add mushroom mixture. Add half of the pesto and a ladle of pasta water. Toss. Add more pesto if needed. Top with a few walnuts and freshly grated Parmesan. Serve with rustic Italian bread.
Save extra pesto by placing in container and covering with a small amount of olive oil or with plastic wrap on top of the pesto to keep out air which will turn it black. Keep in refrigerator and use as a great dip for bread or crackers, or on other dishes.